Examine the thick outer husk of the coconut if it is still attached. The husk of a young coconut is green and turns yellow to dark tan as it ripens. The husk of a mature coconut can be easily peeled away.
Select peeled coconuts with a dark brown shell. The coconut should feel heavy for its size.
Press your thumb gently against the three "eyes" on the bottom of the nut. They should feel dry and slightly soft compared to the hard shell. Check the entire shell for signs of mold.
Hold the coconut to your ear and shake it. In a mature coconut, you will hear the liquid sloshing around freely. The more liquid, the fresher the coconut.